So, what causes MDD? The exact cause is unknown, but research has focused on the balance of certain chemicals called neurotransmitters, specifically serotonin, norepinephrine (also called noradrenaline) and dopamine. These neurotransmitters, especially serotonin, are prevalent in the areas of the brain -- like the limbic system and upper brainstem -- that control mood and emotions. See How Your Brain Works for more information.
Research indicates that patients with MDD either do not have enough serotonin or norepinephrine in these areas of the brain or have an imbalance between the two types of neurotransmitters. Antidepressants are designed to increase the levels of these neurotransmitters in the limbic system. So, to understand how antidepressants work, we must look at the process of neurotransmission.
Your brain and nervous system are made of nerve cells, or neurons. Like wires in your home's electrical system, nerve cells make connections with one another in circuits called neural pathways. Unlike wires in your home, nerve cells do not touch, but they come close together at synapses. At the synapse, the two nerve cells are separated by a tiny gap, or synaptic cleft. The sending neuron is called the presynaptic cell, while the receiving one is called the postsynaptic cell. Nerve cells send chemical messages called neurotransmitters in a one-way direction across the synapse from presynaptic cell to postsynaptic cell.
Next, we'll look at this process using serotonin as an example.